Apparatus for burning biomass material is shown in U.S. Pat. Nos. 1,256,146; 1,862,920; and 783,476. The Ward U.S. Pat. No. 1,256,146 shows a grate system wherein tubular members are inclined downwardly in the direction of fuel travel. The adjacent tubular members comprising the grate system are oscillated oppositely and have threads on their surfaces to promote movement of the fuel downwardly and to cause the ash to sift through to the underside. Air is supplied to the lower ends of the tubular members for cooling. The structure therein shown is objectionable because the oscillation of the tubular members in opposite directions tends to crush and wedge the fuel between the tubular members such that any foreign matter which is present becomes entrained between the tubular members and the screw thread surfaces tend to force the fuel downwardly at a rate which is not consistent with efficient pyrolysis. Ehrlich U.S. Pat. No. 1,812,940 and Smith U.S. Pat. No. 783,746 shows grate structures in the form of hollow grate bars disposed transversely of the direction of flow which neither assists in the downward flow of material nor provides for preheating of the fuel to insure efficient pyrolysis. It is the purpose of this invention to provide a grate structure comprised of hollow grate bars wherein the fuel is allowed to flow freely downwardly thereon as pyrolysis takes place at a rate to assure efficient pyrolysis, to provide for sifting of the ash through the grate bars to the underside without crushing or jamming of fuel between the grate bars and to provide for cooling of the grate bars and preheating the fuel as it descends to enhance pyrolysis.